Spatial inhibition and the visual cortex: a magnetic resonance spectroscopy imaging study.

INTRODUCTION: Deficits in processing spatial information have been observed in clinical populations who have abnormalities within the dopamine (DA) system. As psychostimulants such as methamphetamine (MA) are particularly neurotoxic to the dopaminergic system it was of interest to examine the performance of MA-dependent individuals on a task of spatial attention. METHOD: 51 MA-dependent subjects and 22 age-matched non-substance abusing control subjects were tested on a Spatial Stroop attention test. MR Spectroscopy (MRS) imaging data were analyzed from 32 MA abusers and 13 controls. RESULTS: No group differences in response time or accuracy emerged on the behavioral task with both groups exhibiting equivalent slowing when the word meaning and the spatial location of the word were in conflict. MRS imaging data from the MA abusers revealed a strong inverse correlation between NAA/Cr ratios in the Primary Visual Cortex (PVC) and spatial interference (p=0.0001). Moderate inverse correlations were also seen in the Anterior Cingulate Cortex (ACC) (p=0.02). No significant correlations were observed in the controls, perhaps due to the small sample of imaging data available (n=13). DISCUSSION: The strong correlation between spatial conflict suppression and NAA/Cr levels within the PVC in the MA-dependent individuals suggests that preserved neuronal integrity within the PVC of stimulant abusers may modulate cognitive mechanisms that process implicit spatial information.

Callosal involvement in a lateralized stroop task in alcoholic and healthy subjects.

To investigate the role of interhemispheric attentional processes, 25 alcoholic and 28 control subjects were tested with a Stroop match-to-sample task and callosal areas were measured with magnetic resonance imaging. Stroop color-word stimuli were presented to the left or right visual field (VF) and were preceded by a color cue that did or did not match the word's color. For matching colors, both groups showed a right VF advantage; for nonmatching colors, controls showed a left VF advantage, whereas alcoholic subjects showed no VF advantage. For nonmatch trials, VF advantage correlated with callosal splenium area in controls but not alcoholic subjects, supporting the position that information presented to the nonpreferred hemisphere is transmitted via the splenium to the hemisphere specialized for efficient processing. The authors speculate that alcoholism-associated callosal thinning disrupts this processing route.

Impact of cardiac resynchronization therapy using hemodynamically optimized pacing on left ventricular remodeling in patients with congestive heart failure and ventricular conduction disturbances.

OBJECTIVES: We sought to investigate the impact of six months of cardiac resynchronization therapy (CRT) on echocardiographic variables of left ventricular (LV) function. BACKGROUND: Cardiac resynchronization therapy has recently been introduced as a new therapeutic modality in patients with advanced heart failure (HF) and conduction abnormalities. However, most studies have only investigated the early hemodynamic effects of CRT. METHODS: Twenty-five patients (12 women and 13 men; 59.8 +/- 5.1 years old) with advanced HF caused by ischemic (n = 7) or idiopathic dilated cardiomyopathy (n = 18) and a prolonged QRS complex were analyzed. All patients underwent early hemodynamic testing with a randomized testing protocol; echocardiographic measurements were compared before implantation and after six months of CRT. RESULTS: Left ventricular end-diastolic and end-systolic diameters (LVEDD and LVESD, respectively) were significantly reduced after six months (LVEDD from 71 +/- 10 to 68 +/- 11 mm, p = 0.027; LVESD from 63 +/- 11 to 58 +/- 11 mm, p = 0.007), as were LV end-diastolic and end-systolic volumes (LVEDV from 253 +/- 83 to 227 +/- 112 ml, p = 0.017; LVESV from 202 +/- 79 to 174 +/- 101 ml, p = 0.009). Ejection fraction was significantly increased (from 22 +/- 7% to 26 +/- 9%, p = 0.03). "Nonresponders," with regard to LV volume reduction, had significantly higher baseline LVEDV, compared with "responders" (351 +/- 52 vs. 234 +/- 74 ml, p = 0.018). Overall, there was only mild mitral regurgitation at baseline, with a minor reduction by semiquantitative analysis. The results of early hemodynamic testing did not predict the volume response. CONCLUSIONS: Cardiac resynchronization therapy may lead to a reduction in LV volumes in patients with advanced HF and conduction disturbances. Volume nonresponders have significantly higher baseline LVEDV.

Anterior cingulate metabolism correlates with stroop errors in paranoid schizophrenia patients.

UNLABELLED: Using [O-15]-H(2)O PET Carter et al. (1997) reported that medicated patients with schizophrenia performing computerized single trial Stroop (1935) showed a reduction in the anterior cingulate activation response to the more attention demanding, incongruent Stroop condition. In that study, both patients and controls also showed a direct correlation between anterior cingulate activation and errors committed during incongruent trials of the task. In this study we follow up with an examination of paranoid schizophrenia outpatients and controls with very high resolution positron emission tomography (PET) and the longer half-life tracer [F-18]-fluorinated deoxyglucose (FDG) (Valk et al. 1990). All subjects (10 controls and 9 paranoid schizophrenia patients) were studied with FDG-PET while performing a computerized trial-by-trial version of the Stroop task during the uptake phase of the tracer (Carter et al. 1992). RESULTS: As in previous studies using the single trial Stroop, patients were able to perform the task but made more color-naming errors during incongruent trials than controls. The patients in the present study showed a trend towards increased metabolic activity in the right anterior cingulate cortex. In the patient group, but not in controls, the anterior cingulate glucose metabolic rate correlated positively with the total incongruent trial errors. CONCLUSION: These results are consistent with the hypothesis that the anterior cingulate plays a performance-monitoring role during human cognition. This study does not rule out a reduction in error sensitivity in this region of the brain in schizophrenia, as other studies have suggested, however the data show that in unmedicated patients with the paranoid subtype this function is preserved to some extent.

Interpreting Stroop interference: an analysis of differences between task versions.

The present study investigated methodological differences between the clinical version of the Stroop Color and Word Test and the computerized single-trial version. Three experiments show that different presentations of the Stroop task can produce different levels of interference. The 1st experiment examined the effect of blocking; the 2nd experiment examined different control conditions. Greater interference in the blocked clinical version appears to result from lower response times (RTs) in the neutral condition, not from greater RTs in the incongruent condition. Experiment 3 examined the impact of shifting attention across locations while responding to Stroop stimuli. The present set of findings sheds light on the inconsistency in the clinical literature and demonstrates that the method and selection of neutral stimuli (that provide the baseline by which interference is measured) are critical because they clearly can change performance.

Flow cytometric analysis of group B streptococci phagocytosis and oxidative burst in human neutrophils and monocytes.

Group B streptococci (GBS) are a major cause of meningitis and septicemia in neonates and numerous invasive diseases in adults. Host defense against GBS infections relies upon phagocytosis and killing by phagocytic cells. To better understand the importance of this defense mechanism a flow cytometric assay was developed to study phagocytosis and oxidative burst of leukocytes stimulated by bacteria. GBS labeled with fluorescein isothiocyanate were used for phagocytosis experiments and the extracellular fluorescence was quenched by ethidium bromide to differentiate intracellular from extracellular bacteria. The intracellular oxidative burst was determined by using 2',7'-dichlorofluorescein diacetate to measure hydrogen peroxide production and hydroethidine for superoxide anion production. We found that for GBS serotypes Ia, Ib/c, II, and III phagocytosis was greater in neutrophils than monocytes. Hydrogen peroxide production and superoxide anion production were also greater for neutrophils than monocytes in all serotypes tested. A comparison of seven type III strains revealed greater phagocytosis and superoxide anion production by neutrophils than monocytes but no difference in hydrogen peroxide production. Therefore, monocytes react similarly as neutrophils in response to GBS but at a reduced level. This methodology of measuring both phagocytosis of GBS and oxidative burst simultaneously in neutrophils and monocytes should be very useful in further studies on the importance of factors such as complement and IgG receptors for the killing of bacteria.

Effect of pacing chamber and atrioventricular delay on acute systolic function of paced patients with congestive heart failure. The Pacing Therapies for Congestive Heart Failure Study Group. The Guidant Congestive Heart Failure Research Group.

BACKGROUND: Previous studies of pacing therapy for dilated congestive heart failure (CHF) have not established the relative importance of pacing site, AV delay, and patient heterogeneity on outcome. These variables were compared by a novel technique that evaluated immediate changes in hemodynamic function during brief periods of atrial-synchronous ventricular pacing. METHODS AND RESULTS: Twenty-seven CHF patients with severe left ventricular (LV) systolic dysfunction and LV conduction disorder were implanted with endocardial pacing leads in the right atrium and right ventricle (RV) and an epicardial lead on the LV and instrumented with micromanometer catheters in the LV, aorta, and RV. Patients in normal sinus rhythm were stimulated in the RV, LV, or both ventricles simultaneously (BV) at preselected AV delays in a repeating 5-paced/15-nonpaced beat sequence. Maximum LV pressure derivative (LV+dP/dt) and aortic pulse pressure (PP) changed immediately at pacing onset, increasing at a patient-specific optimal AV delay in 20 patients with wide surface QRS (180+/-22 ms) and decreasing at short AV delays in 5 patients with narrower QRS (128+/-12 ms) (P<0.0001). Overall, BV and LV pacing increased LV+dP/dt and PP more than RV pacing (P<0.01), whereas LV pacing increased LV+dP/dt more than BV pacing (P<0.01). CONCLUSIONS: In this population, CHF patients with sufficiently wide surface QRS benefit from atrial-synchronous ventricular pacing, LV stimulation is required for maximum acute benefit, and the maximum benefit at any site occurs with a patient-specific AV delay.

The effects of antipsychotic medication on sequential inhibitory processes.

The authors used a Stroop negative priming paradigm to examine the effects of antipsychotic medication on selective attentional processes. The performance of 14 patients with schizophrenia who were withdrawn from neuroleptic medication was compared with that of 10 medicated patients and 16 matched controls. Results demonstrated an increase in negative priming to normal levels with neuroleptic therapy. In contrast, within-trial interference and facilitation effects appeared to be less sensitive to medication therapy. The sustainment of inhibitory processes over time may differentiate the inhibitory mechanisms of the medication-withdrawn patients from both the medicated patients and the matched controls. The study of sequential inhibitory processes and their response to neuroleptic treatment could be important methods for understanding the temporal parameters associated with inhibition in schizophrenia.

Enhancement of bismuth antibacterial activity with lipophilic thiol chelators.

The antibacterial properties of bismuth are greatly enhanced when bismuth is combined with certain lipophilic thiol compounds. Antibacterial activity was enhanced from 25- to 300-fold by the following seven different thiols, in order of decreasing synergy: 1,3-propanedithiol, dimercaprol (BAL), dithiothreitol, 3-mercapto-2-butanol, beta-mercaptoethanol, 1-monothioglycerol, and mercaptoethylamine. The dithiols produced the greatest synergy with bismuth at optimum bismuth-thiol molar ratios of from 3:1 to 1:1. The monothiols were generally not as synergistic and required molar ratios of from 1:1 to 1:4 for optimum antibacterial activity. The most-active mono- or dithiols were also the most soluble in butanol. The intensity of the yellow formed by bismuth-thiol complexes reflected the degree of chelation and correlated with antibacterial potency at high molar ratios. The bismuth-BAL compound (BisBAL) was active against most bacteria, as assessed by broth dilution, agar diffusion, and agar dilution analyses. Staphylococci (MIC, 5 to 7 microM Bi3+) and Helicobacter pylori (MIC, 2.2 microM) were among the most sensitive bacteria. Gram-negative bacteria were sensitive (MIC, < 17 microM). Enterococci were relatively resistant (MIC, 63 microM Bi3+). The MIC range for anaerobes was 15 to 100 microM Bi3+, except for Clostridium difficile (MIC, 7.5 microM). Bactericidal activity averaged 29% above the MIC. Bactericidal activity increased with increasing pH and/or increasing temperature. Bismuth-thiol solubility, stability, and antibacterial activity depended on pH and the bismuth-thiol molar ratio. BisBAL was stable but ineffective against Escherichia coli at pH 4. Activity and instability (reactivity) increased with increasing alkalinity. BisBAL was acid soluble at a molar ratio of greater than 3:2 and alkaline soluble at a molar ratio of less than 2:3. In conclusion, certain lipophilic thiol compounds enhanced bismuth antibacterial activity against a broad spectrum of bacteria. The activity, solubility, and stability of BisBAL were strongly dependent on the pH, temperature, and molar ratio. Chelation of bismuth with certain thiol agents enhanced the solubility and lipophilicity of this cationic heavy metal, thereby significantly enhancing its potency and versatility as an antibacterial agent.

Problems and pitfalls in evaluating studies for pacing in heart failure.

Pacing therapy has been recently proposed as a new non-pharmacological approach to patients suffering from congestive heart failure refractory to medical therapy (refractory CHF), but the extention and the real benefit of this method remains to be defined. Although pacing therapy for refractory CHF has been restricted to patients in sinus rhythm presenting atrial, atrioventricular or interventricular conduction disturbances, considerable conflicting results have been published. The contradictory data is most likely due to large heterogeneity of the investigated study population (ie etiology of CHF, NYHA class, duration of follow-up, end-points of the study, etc.), to difference in study methology as well as in the site and modality of acute and chronic pacing. Although several empirical data indicates, at least in some individuals, major improvement in CHF symptoms by properly coordinating the atria and the ventricles thus reducing pre-systolic mitral and/or tricuspidal regurgitation or finally, prolonging the diastolic filling time, a lack of understanding of the mechanisms responsible for acute and chronic benefit persists. In evaluating pacemaker therapy as a new supportive treatment for CHF, the clinical investigator must consider that each study protocol embodies assumptions and methodological limitations and thus provides an incomplete analysis of potential benefit. Relying solely on noninvasive measures is risky due to problems of sensitivity and repeatability.

Acute hemodynamic improvement by pacing in patients with severe congestive heart failure.

Since the first report on dual chamber pacing for congestive heart failure (CHF) in 1991, a number of investigators have explored the topic with conflicting results. These conflicts may arise from an incomplete understanding of the mechanisms by which pacing improves cardiac function. Potential mechanisms include: (1) increase in filling time; (2) decrease in mitral regurgitation; (3) optimization of left heart mechanical atrioventricular delay (left heart MAVD); and (4) normalization of ventricular activation. One or more of these mechanisms may be operative in an individual patients, implying that patients may require individual optimization. Acute pacing studies were conducted on nine CHF patients, NYHA Class II-III to Class IV. Measurements of conduction times in sinus rhythm revealed: (1) normal interatrial conduction times (59 +/- 5 ms) in all patients, with wide variations in interventricular conduction times (range, -5-105 ms); and (2) a wide range of left heart MAVD (range, 97-388 ms). While pacing the right, left, or both ventricles, measurement of high fidelity aortic pressure and mitral and aortic velocities revealed the following: (1) 6 of 9 patients increased mean pulse pressure over sinus during RV or LV pacing at an optimal AV delay; (2) the maximum aortic pulse pressure was achieved when the atrium was not paced: an 8% increase over sinus pulse pressure with paced RV versus a 5% decrease for paced atrium and RV at optimum AV delay (paired Student's t-test, P = 0.01), and a 0% increase over sinus with paced LV versus 7% decrease for paced atrium and LV at optimum AV delay, P < 0.05; (3) significant dependence on pacing site was noted, with 4 patients doing best with RV pacing, 3 patients achieving a maximum with LV pacing, and 2 patients showing no preference; and (4) 2 of 4 patients with restrictive filling patterns were converted to nonrestrictive patterns with optimum pacing. Patient hemodynamics appear to benefit acutely from individually optimized pacing. Increases in filling time, optimization of left heart MAVD, and normalization of intraventricular activation are the most significant mechanisms. Atrial pacing is inferior to atrial sensed modes if the patient has a functional sinus node.

Child Behaviour Checklist classification of behaviour disorder.

OBJECTIVE: The aim of this study was to determine the applicability of the published clinical cut-off scores of the Child Behaviour Checklist (CBCL) for the classification of behaviour disorders. METHODOLOGY: Child Behaviour Checklists were obtained for 1342 subjects newly referred to the six major mental health centres in Melbourne. The normative community sample of 1002 7-, 12- and 15-year-olds was drawn from a school-based asthma prevalence study. RESULTS: The mean total problem T-score for the children referred to mental health centres was 67 and was above the clinical range for all age groups. Using referral to psychiatric services as the gold standard, the sensitivity and specificity of the CBCL using a cut-off of > or = 60, was 77.4 and 83.2%, respectively. This compares favourably with the sensitivity of 68% and specificity of 82% for the American sample. Using a cut-off score of > or = 63, the sensitivity was 70.5% and the specificity was 88.6%. The referred and community samples differed with respect to socio-economic status, family structure and mothers' level of education. Fifty-two per cent of the clinically referred children lived with both parents, compared with 89% of the community sample. CONCLUSIONS: While there are some limitations to this study in terms of both the clinic and community sample, support is provided for the usefulness and applicability of the recommended CBCL cut-off scores in an Australian population.

Normal sustained effects of selective attention are absent in schizophrenic patients withdrawn from medication.

Sustained attentional deficits have been widely reported in groups of medicated schizophrenic patients, but less is known about sequential attentional processes in patients withdrawn from medication. The attentional performance of 12 medication-withdrawn schizophrenic outpatients was compared with that of 16 matched normal volunteers on a Stroop negative priming task. This task allowed examination of both within-trial and between-trial attentional effects. Compared with the volunteers, the medication-withdrawn schizophrenic patients showed normal within-trial attentional effects as measured by standard Stroop interference and facilitation. Across trials, however, the schizophrenics exhibited reduced negative priming compared with the volunteers and in some cases a complete reversal of sustained inhibitory processes. The findings suggest that a normal inhibitory tag occurred during initial selection in the patient group, but it did not influence a subsequent act of selection as was the case for the normal volunteers. Either inhibition decayed at an abnormally fast rate in the patient group or a separate facilatory tag dominated. In either case, priming effects linked to attentional selection were clearly abnormal in the medication-withdrawn patient group.

Salicylate-enhanced exposure of Klebsiella pneumoniae subcapsular components.

The capsular polysaccharide (CPS) of Klebsiella pneumoniae is an important virulence factor. Salicylate, which inhibits CPS production, was used to expose subcapsular antigens and components that may play an important role in host defense. Salicylate treatment greatly increased phagocytosis of five O1 serotypes by human polymorphonuclear leukocytes with normal rabbit serum and rabbit antisera against purified O1 lipopolysaccharide (O1LPS) as opsonins (p < 0.01 or < 0.05). Similar results were obtained with rabbit antiserum against a non-encapsulated isogenic strain. To further determine how salicylate increases susceptibility to phagocytosis, the binding of monoclonal antibodies against O1LPS or the LPS core and the binding of complement component C3b were measured by ELISA. The data indicate that salicylate reduced the barrier of CPS in serotypes O1:K1, O1:K10, and O1:K16 and unmasked subcapsular antigenic components in serotypes O1:K2 and O1:K66 so that bound opsonins could react with receptors on phagocytes. Serum bactericidal assays supported this conclusion. Therefore, decapsulating agents such as salicylate accentuate phagocytosis of K. pneumoniae by making subcapsular antigens and components accessible to immune and nonimmune host defences and vaccination with subcapsular antigens may exhibit optimal protection against lethal infection when combined with salicylate therapy.

Polysaccharide capsule-mediated resistance to opsonophagocytosis in Klebsiella pneumoniae.

The polysaccharide capsule of Klebsiella pneumoniae is an important virulence factor that confers resistance to phagocytosis. The treatment of encapsulated bacteria with salicylate to inhibit capsule expression was found to enhance the phagocytosis of encapsulated bacteria by human neutrophils only in the presence of cell surface-specific antibodies. Both type-specific rabbit antisera and anticapsular human hyperimmune globulin were employed as opsonins. Salicylate significantly enhanced phagocytosis with homologous, but not heterologous, whole-cell antisera. Antisera, diluted 1:40, no longer opsonized fully encapsulated bacteria but promoted the uptake of multiple salicylate-treated bacteria in > 90% of neutrophils. Salicylate (0.25 to 1.0 mM) also enhanced opsonization with globulin against homologous bacteria. Higher salicylate levels (1 to 2.5 mM) enhanced the opsonization of heterologous serotypes with human globulin. The nature of antibody attachment to encapsulated bacteria was determined by immunofluorescence. Even after the addition of purified capsular polysaccharide to prevent phagocytosis, K-specific antibodies attached in large amounts to bacteria. K-specific antibodies reacted with antigens throughout the capsule and showed a predilection for a denser inner layer of the capsule, indicating that many of the K-specific antibodies may be masked underneath the capsule surface. K-specific antibodies can also be rendered nonfunctional by soluble, cell-free capsular antigen. In culture, large quantities of soluble capsular polysaccharide extrude from bacteria after overnight growth. The reduction in capsule expression caused by salicylate largely affected the soluble, cell-free fraction. Purified capsular polysaccharide was shown to retard the opsonophagocytosis of salicylate-treated bacteria in a concentration-dependent manner. However, extensive washing of encapsulated bacteria to remove loosely attached capsular material did not significantly enhance opsonophagocytosis. In conclusion, cell-free capsule and cell-associated capsule are antiphagocytic; both act to neutralize K-specific antibodies by binding or concealment. Salicylate-mediated inhibition of capsule expression, particularly of the cell-free fraction, improved K-specific opsonization dramatically.

Effects of acute changes in canine LV-chamber volume and shape on accuracy of impedance catheter estimates of LV-chamber volume.

The accuracy with which a multiple-electrode impedance catheter (IC) tracks instantaneous global, in-situ left ventricular (LV) volumes was tested in 13 anesthetized dogs scanned in the Dynamic Spatial Reconstructor (DSR), a fast volumetric computed tomographic (CT) scanner. All dogs were scanned during control conditions and during an acute hemodynamic intervention. Hypertonic saline calibrations were performed for the IC prior to each DSR scan. In six of the dogs the IC-derived LV end-diastolic volume (Y) correlated with the DSR-derived global LV end-diastolic volume (X) as follows: end-diastolic volume, Y = 1.01X - 9.9, r = 0.812. The IC-derived LV end-diastolic volume, under control conditions, correlated with the DSR-derived truncated (i.e., that region of the LV chamber between the proximal and distal electrodes of the IC catheter) LV end-diastolic volume, Y = 1.00X + 17.4, r = 0.803. Under reduced preload the relation was Y = 1.3X - 15.26, r = 0.911. The segmental volume (between adjacent sensing electrodes on the IC) at the basal portion of the LV correlated poorly (Y = 1.88X + 3.3, r = 0.459 etc.), but correlated better at mid- and more apical LV levels (Y = 0.97X + 2.7, r = 0.762). Correlations between segmental stroke volumes were similar at basal (Y = 1.31X + 1.60, r = 0.815) and mid- and more apical levels (Y = 1.42X + 0.11, r = 0.763). Stroke volume during acute ischemia (two dogs) was Y = 1.33X - 1.41, r = 0.717; during acutely decreased preload (four dogs) it was Y = 1.24X - 2.88, r = 0.572). Thus, the IC tracks the changes in LV-chamber volume throughout a cardiac cycle quite well under a variety of conditions, but accuracy deteriorates as the shape of the LV chamber changes in response to changes in hemodynamic loading or local myocardial ischemia.

Improvement of cardiac function in patients with severe congestive heart failure and coronary artery disease by dual chamber pacing with shortened AV delay.

Medical therapy often fails to control symptoms of severe heart failure. The possibility of modifying to some degree the global ventricular performance with the implantation of a physiological dual chamber pacemaker, set with a short atrioventricular delay (100 msec), has been adopted in two patients with severe heart failure due to coronary artery disease. The baseline clinical condition of both patients was characterized by leg edema, ascites, dyspnea at rest, or even orthopnea with a functional New York Heart Association (NYHA) class III-IV. Acute measurements of hemodynamic and echocardiographic parameters during stepwise shortening of AV interval guided the pacemaker implantation and setting of AV delay in the chronic phase. Within a few days after pacemaker implantation, both patients considerably improved their clinical status as well as their functional NYHA class, improving to class II in one patient and to class II-III in the other patient. In addition, modification of systolic and diastolic parameters paralleled these improvements functional class and clinical condition. Pacemaker therapy in severe heart failure refractory to medical therapy can be of considerable benefit in patients whose quality-of-life is severely compromised when pharmacological therapy is no longer effective. Acute hemodynamic and echocardiographic testing is useful in assessing the most appropriate AV delay and pacing mode.

Improvement in intracardiac impedance volumes by field extrapolation.

The measurement of volume by electrical impedance is complicated by non-homogeneous current distribution resulting from small current sources, by the irregular shape of the ventricle, and by loss of current to surrounding anatomical structures. A mathematical technique, field extrapolation, was developed to partially correct the current distribution. The technique mathematically transforms measured potentials into the potential distribution which would result from infinitely distant current sources. The linear correlation coefficient between impedance stroke volume or cardiac output using field extrapolation and thermodilution stroke volume or cardiac output was 0.83 (n = 86) in 11 dogs and 0.76 (n = 92) in 12 patients. The average linear correlation coefficient between impedance stroke volume and integrated aortic flow in four dogs was 0.83 +/- 0.09 (n = 49) using field extrapolation.